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Lowest RPM of an electric bike motor?

What is the lowest possible RPM of an electric bike motor?  It seems to be the cheapest alternative to a slow gearmotor.  Thanks.

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A lot will depend on the internal design of the motor, the voltage you run it on and how you drive it. Theeoretically you can run it as slow as you want. In practice the coils are only designed to accept a short burts of power. At lower speeds the inductive reactance of the windings drops to zero and you end up with a short circuit. 

By feeding the windings with a 50% dutycycle / variable frequency you can maintain high inductive reactance even when the wheel is stationary. The frequency will determine the maximum current that can flow. My experience indicates that frequencies of 100KHz and higher may be required if you want the motor to hold a position (like a stepper motor) without damaging the windings.